Fixed opening the SSH firewall port

[lslezak]

Problem

• The firewall configuration is not handled properly when the firewall is not installed. That can happen when the "onlyRequires" flag is set in the software configuration.
• I overlooked that the command is running in the chroot wrapper, the code actually tested the failure for missing chroot command instead.

Solution

• The chroot command returns exit status 127 when the command to run does not exist. If that value is returned then ignore the error.